The present invention relates to a method for molding a fiber-impregnated sheet molding compound (hereinafter, "sheet molding compound" is referred to as SMC) to produce moldings of a solid shape. It also relates to the moldings produced by this method.
In order to produce moldings having a solid shape by using a SMC, a method has so far been used, wherein a SMC of high viscosity is cut into a proper size, this cut sheet is introduced into a mold for forming, composed of a male and female pair of molds, and high temperature and high pressure is imposed thereon, whereby forming a defined shape by proper shaping and curing.
However, this method is highly expensive because of the complexity and expensiveness of the metal mold and the requirement for high pressure equipment. Therefore, it is economically disadvantageous. Also, molding under pressure requires a long time, so that it is difficult for the method to respond quickly to a request for a particular product. This long molding time, accompanied by diversification of product needs, increasing variety of small quantities in a given order and shortening of the time limit demanded for delivery is difficult to achieve.
On the other hand, to solve the defect in the above method, a method has been proposed in the Japanese official patent provisional publication (showa 61-205121), wherein a pre-preg sheet, which is not yet hardened, and is composed of an SMC, is adhered under vacuum along a female mold for shaping, and then is hardened by irradiating with ultraviolet light. Since this method uses only a female mold as a mold for forming, does not require such accurate mold precision, and does not need high pressure and high temperature, there are benefits to be derived, such as inexpensive cost for equipment and a relatively simple molding process, so that efficiency increases in the molding operation can be achieved.
There is, however, a problem in the prior art, in that the thickness of the shaped pre-preg sheet, viz. a moldings, locally differs as a function of the time which, after the vacuum-adsorption operation is initiated, is needed to bring the pre-preg sheet into contact with the female mold.
FIG. 8 shows a typical conventional method for molding. A pre-preg sheet is arranged on an upper face of a female mold m. That is, the sheet for forming S is sucked, by reducing pressure through suction holes h which open on a mold face of the female mold m, onto the female mold. A central part of the sheet for forming S is at first sucked into the center of the female mold m. After the central part of the sheet for forming S comes into contact with the mold face, a circumferencial part of the sheet gradually undergoes the shaping in contact with the mold face and, at this time, in the central part which has at first come into contact with the mold face, the resin material is carried away toward the circumferencial part by back pressure which is accompanied by the vacuum adherence, so that the sheet for forming becomes thinner in this central part. In addition to decreasing the thickness of the central part of the sheet for forming as mentioned above, the thickness of the sheet for forming S, when compared with what would have been expected, locally differs in that the resin material of the sheet for forming S flows or is unevenly distributed as a function of the unevenness and inclination of a mold face of the female mold.
In a method wherein the forming is carried out at high temperature and high pressure under conditions such that a material for forming is disposed between a male and female pair of molds for forming, a molding which exactly corresponds to the shape of the male and female molds for forming is obtained and, therefore, a problem of the above kind does not take place. However, in a method wherein the sheet for forming S takes a shape along the mold for forming by using either one only of a male or a female mold for forming, the shape and thickness of the sheet for forming S can not be controlled, at the side of the sheet for forming S which is not in contact with a mold face, so that a deviation in thickness as mentioned above inevitably occurs.
If the sheet for forming S, after shaping of the above kind, that is after molding, shows a difference in thickness when compared with the expected value, mechanical strength may decrease where the thickness is thin or the moldings may lack their expected properties. Therefore, in a method of said conventional art, moldings of fiber-reinforced plastics (hereinafter referred to as FRP), which are controlled as to their desired thickness, in order to get desired properties, have not been obtained.